This invention relates to a multiple-valve internal combustion engine for a motorized two-wheeled vehicle or the like wherein three intake valves and two exhaust valves are provided for a common cylinder.
Engines of this kind, having multiple valves are known such as the one shown in FIGS. 1 and 2 and disclosed, for instance, in Japanese Unexamined Patent Application Publication No. Sho 57-183553. In such an engine, the cylinder head a is provided with two intake valves c, c at its portion on one side on a circumference surrounding the center line of a combustion chamber b and with two exhaust valves d, d at its portion on the other side on the same circumference. In addition, another intake valve c is provided at its portion on the center line of the combustion chamber b. The engine is provided with the three intake valves c, c, c in total for improving the output thereof.
This engine, however, has the disadvantage that, because of the arrangement that the third intake valve c is provided at the center position of the cylinder head s, the position for providing an ignition plug e has to be deviated radially from the center position. In order to obtain uniform combustion, a pair of ignition plugs e have to be disposed on both outsides of the center position. This arrangement is troublesome. In addition, for avoiding interference with the ignition plugs e, e, a pair of cam shafts f, f have to be horizontally disposed on both lateral outsides of a disposition section of the ignition pliug e, e. Thus, it cannot be avoided that the engine is formed into a so called "DOHC" type which is disadvantageous. Additionally, this type is also inconvenient in that, due to the arrangement that those valves are disposed so that the intake valves may be in parallel one with another in their inclined lines and the exhaust valves may be in parallel one with another in their inclined axial lines, the end surfaces of the respective valves crossing at right angles to their respective stepped shoulders protruded from a top surface wall of the combustion chamber. This forms the upper surface wall of the combustion chamber into a rugged surface which is liable to cause an ununiform combustion resulting in lowering in the combustibility.
There has been also known an engine, as disclosed in U.S. Pat. No. 2,710,602, for instance, wherein the cylinder head in provided at its portion on one side on the circumference surrounding the center line of the combustion chamber with three intake valves and at its portion on the other side on the circumference with two exhaust valves. A single ignition plug is provided at its portion on the center line of the combustion chamber.
This type of engine, however, is also inconvenient in that, because the ignition plug is provided vertically along the center line of the cylinder, the engine is required to be formed into a DOHC type in order to avoid interference with those valves. Additionally, due to such arrangement that the top wall surface of the combustion chamber is formed into a horizontal place crossing at right angles the axial line of the cylinder and the respective valves are all provided vertically in parallel with the cylinder axial line, the cylinder must be enlarged in its bore diameter from a view-point of layout of the valves.